The 'Tonight Show' host hit the stage for an unexpected special appearance in NYC.
The Jonas Brothers kicked off their new world tour this weekend -- called simply The Tour -- at the iconic baseball stadium, and for their second sold-out show, they were joined by the Tonight Show host.
Notably, Fallon came out sporting the scruffy beard he's grown since his talk show has been on hiatus amid the ongoing WGA/SAG-AFTRA strikes, as well as some moody dark shades to complete the rocker ensemble.
Also notably, Fallon came out to belt out a lively cover of "Mr. Brightside" -- a song by The Killers. So his special appearance at a Jonas Brothers show didn't even see him sing a Jonas Brothers tune.
Needless to say, fans were equal parts enthralled by the surprise guest appearance and somewhat baffled.
"Jimmy Fallon just randomly showed up at Jonas Brothers Yankee Stadium Night 2 and is leading the crowd in 'Mr. Brightside,'" one bemused concertgoer wrote on Twitter, alongside a video of the comedian's enthusiastic cover.
In one video captured by another fan, Fallon can be heard asking the crowd, "How would you all like to be in the world's biggest karaoke party for two minutes?" The question was met with a raucous round of cheers.
Fallon's appearance was about as unexpected as their surprise guest on Saturday, during their opening night when they were joined on stage by gospel icon Kirk Franklin.
Meanwhile, Nick, Joe and Kevin Jonas stopped by for ET's "Spilling the E-Tea" last week to dish on their new tour, and the artists all said they were ready and excited to kick things off.
"I can't wait," Nick said. "To put it simply, I can't wait."
Meanwhile, Joe admitted that he'd been feeling a bit "overwhelmed" leading to their shows at Yankee Stadium, where they will be performing tunes from across five different albums.
"But now that we've gotten it up and running and we did a dress rehearsal with a live audience, two nights ago, I think we finally realized we can do this thing," Joe said. "We built an incredible show for the people out there that have question marks in their brains saying, 'How are these boys going to do this?' Well, we can and we will. We'll see you at Yankees."
Check out the video below to hear more.